Internet Engineering Task Force (IETF) E. Rescorla
Request for Comments: 6347 RTFM, Inc.
Obsoletes: 4347 N. Modadugu
Category: Standards Track Google, Inc.
ISSN: 2070-1721 January 2012
Datagram Transport Layer Security Version 1.2
Abstract
This document specifies version 1.2 of the Datagram Transport Layer
Security (DTLS) protocol. The DTLS protocol provides communications
privacy for datagram protocols. The protocol allows client/server
applications to communicate in a way that is designed to prevent
eavesdropping, tampering, or message forgery. The DTLS protocol is
based on the Transport Layer Security (TLS) protocol and provides
equivalent security guarantees. Datagram semantics of the underlying
transport are preserved by the DTLS protocol. This document updates
DTLS 1.0 to work with TLS version 1.2.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6347.
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RFC 6347 DTLS January 2012
Copyright Notice
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Rescorla & Modadugu Standards Track [Page 2]

RFC 6347 DTLS January 20121. Introduction
TLS [TLS] is the most widely deployed protocol for securing network
traffic. It is widely used for protecting Web traffic and for e-mail
protocols such as IMAP [IMAP] and POP [POP]. The primary advantage
of TLS is that it provides a transparent connection-oriented channel.
Thus, it is easy to secure an application protocol by inserting TLS
between the application layer and the transport layer. However, TLS
must run over a reliable transport channel -- typically TCP [TCP].
Therefore, it cannot be used to secure unreliable datagram traffic.
An increasing number of application layer protocols have been
designed that use UDP transport. In particular, protocols such as
the Session Initiation Protocol (SIP) [SIP] and electronic gaming
protocols are increasingly popular. (Note that SIP can run over both
TCP and UDP, but that there are situations in which UDP is
preferable.) Currently, designers of these applications are faced
with a number of unsatisfactory choices. First, they can use IPsec
[RFC4301]. However, for a number of reasons detailed in [WHYIPSEC],
this is only suitable for some applications. Second, they can design
a custom application layer security protocol. Unfortunately,
although application layer security protocols generally provide
superior security properties (e.g., end-to-end security in the case
of S/MIME), they typically require a large amount of effort to design
-- in contrast to the relatively small amount of effort required to
run the protocol over TLS.
In many cases, the most desirable way to secure client/server
applications would be to use TLS; however, the requirement for
datagram semantics automatically prohibits use of TLS. This memo
describes a protocol for this purpose: Datagram Transport Layer
Security (DTLS). DTLS is deliberately designed to be as similar to
TLS as possible, both to minimize new security invention and to
maximize the amount of code and infrastructure reuse.
DTLS 1.0 [DTLS1] was originally defined as a delta from [TLS11].
This document introduces a new version of DTLS, DTLS 1.2, which is
defined as a series of deltas to TLS 1.2 [TLS12]. There is no DTLS
1.1; that version number was skipped in order to harmonize version
numbers with TLS. This version also clarifies some confusing points
in the DTLS 1.0 specification.
Implementations that speak both DTLS 1.2 and DTLS 1.0 can
interoperate with those that speak only DTLS 1.0 (using DTLS 1.0 of
course), just as TLS 1.2 implementations can interoperate with
previous versions of TLS (see Appendix E.1 of [TLS12] for details),
with the exception that there is no DTLS version of SSLv2 or SSLv3,
so backward compatibility issues for those protocols do not apply.
Rescorla & Modadugu Standards Track [Page 4]

RFC 6347 DTLS January 20121.1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [REQ].
2. Usage Model
The DTLS protocol is designed to secure data between communicating
applications. It is designed to run in application space, without
requiring any kernel modifications.
Datagram transport does not require or provide reliable or in-order
delivery of data. The DTLS protocol preserves this property for
payload data. Applications such as media streaming, Internet
telephony, and online gaming use datagram transport for communication
due to the delay-sensitive nature of transported data. The behavior
of such applications is unchanged when the DTLS protocol is used to
secure communication, since the DTLS protocol does not compensate for
lost or re-ordered data traffic.
3. Overview of DTLS
The basic design philosophy of DTLS is to construct "TLS over
datagram transport". The reason that TLS cannot be used directly in
datagram environments is simply that packets may be lost or
reordered. TLS has no internal facilities to handle this kind of
unreliability; therefore, TLS implementations break when rehosted on
datagram transport. The purpose of DTLS is to make only the minimal
changes to TLS required to fix this problem. To the greatest extent
possible, DTLS is identical to TLS. Whenever we need to invent new
mechanisms, we attempt to do so in such a way that preserves the
style of TLS.
Unreliability creates problems for TLS at two levels:
1. TLS does not allow independent decryption of individual
records. Because the integrity check depends on the sequence
number, if record N is not received, then the integrity check
on record N+1 will be based on the wrong sequence number and
thus will fail. (Note that prior to TLS 1.1, there was no
explicit IV and so decryption would also fail.)
2. The TLS handshake layer assumes that handshake messages are
delivered reliably and breaks if those messages are lost.
The rest of this section describes the approach that DTLS uses to
solve these problems.
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RFC 6347 DTLS January 20123.1. Loss-Insensitive Messaging
In TLS's traffic encryption layer (called the TLS Record Layer),
records are not independent. There are two kinds of inter-record
dependency:
1. Cryptographic context (stream cipher key stream) is retained
between records.
2. Anti-replay and message reordering protection are provided by a
MAC that includes a sequence number, but the sequence numbers
are implicit in the records.
DTLS solves the first problem by banning stream ciphers. DTLS solves
the second problem by adding explicit sequence numbers.
3.2. Providing Reliability for Handshake
The TLS handshake is a lockstep cryptographic handshake. Messages
must be transmitted and received in a defined order; any other order
is an error. Clearly, this is incompatible with reordering and
message loss. In addition, TLS handshake messages are potentially
larger than any given datagram, thus creating the problem of IP
fragmentation. DTLS must provide fixes for both of these problems.
3.2.1. Packet Loss
DTLS uses a simple retransmission timer to handle packet loss. The
following figure demonstrates the basic concept, using the first
phase of the DTLS handshake:
Client Server
------ ------
ClientHello ------>
X<-- HelloVerifyRequest
(lost)
[Timer Expires]
ClientHello ------>
(retransmit)
Once the client has transmitted the ClientHello message, it expects
to see a HelloVerifyRequest from the server. However, if the
server's message is lost, the client knows that either the
ClientHello or the HelloVerifyRequest has been lost and retransmits.
When the server receives the retransmission, it knows to retransmit.
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RFC 6347 DTLS January 2012
The server also maintains a retransmission timer and retransmits when
that timer expires.
Note that timeout and retransmission do not apply to the
HelloVerifyRequest, because this would require creating state on the
server. The HelloVerifyRequest is designed to be small enough that
it will not itself be fragmented, thus avoiding concerns about
interleaving multiple HelloVerifyRequests.
3.2.2. Reordering
In DTLS, each handshake message is assigned a specific sequence
number within that handshake. When a peer receives a handshake
message, it can quickly determine whether that message is the next
message it expects. If it is, then it processes it. If not, it
queues it for future handling once all previous messages have been
received.
3.2.3. Message Size
TLS and DTLS handshake messages can be quite large (in theory up to
2^24-1 bytes, in practice many kilobytes). By contrast, UDP
datagrams are often limited to <1500 bytes if IP fragmentation is not
desired. In order to compensate for this limitation, each DTLS
handshake message may be fragmented over several DTLS records, each
of which is intended to fit in a single IP datagram. Each DTLS
handshake message contains both a fragment offset and a fragment
length. Thus, a recipient in possession of all bytes of a handshake
message can reassemble the original unfragmented message.
3.3. Replay Detection
DTLS optionally supports record replay detection. The technique used
is the same as in IPsec AH/ESP, by maintaining a bitmap window of
received records. Records that are too old to fit in the window and
records that have previously been received are silently discarded.
The replay detection feature is optional, since packet duplication is
not always malicious, but can also occur due to routing errors.
Applications may conceivably detect duplicate packets and accordingly
modify their data transmission strategy.
4. Differences from TLS
As mentioned in Section 3, DTLS is intentionally very similar to TLS.
Therefore, instead of presenting DTLS as a new protocol, we present
it as a series of deltas from TLS 1.2 [TLS12]. Where we do not
explicitly call out differences, DTLS is the same as in [TLS12].
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RFC 6347 DTLS January 20124.1. Record Layer
The DTLS record layer is extremely similar to that of TLS 1.2. The
only change is the inclusion of an explicit sequence number in the
record. This sequence number allows the recipient to correctly
verify the TLS MAC. The DTLS record format is shown below:
struct {
ContentType type;
ProtocolVersion version;
uint16 epoch; // New field
uint48 sequence_number; // New field
uint16 length;
opaque fragment[DTLSPlaintext.length];
} DTLSPlaintext;
type
Equivalent to the type field in a TLS 1.2 record.
version
The version of the protocol being employed. This document
describes DTLS version 1.2, which uses the version { 254, 253 }.
The version value of 254.253 is the 1's complement of DTLS version
1.2. This maximal spacing between TLS and DTLS version numbers
ensures that records from the two protocols can be easily
distinguished. It should be noted that future on-the-wire version
numbers of DTLS are decreasing in value (while the true version
number is increasing in value.)
epoch
A counter value that is incremented on every cipher state change.
sequence_number
The sequence number for this record.
length
Identical to the length field in a TLS 1.2 record. As in TLS 1.2,
the length should not exceed 2^14.
fragment
Identical to the fragment field of a TLS 1.2 record.
DTLS uses an explicit sequence number, rather than an implicit one,
carried in the sequence_number field of the record. Sequence numbers
are maintained separately for each epoch, with each sequence_number
initially being 0 for each epoch. For instance, if a handshake
message from epoch 0 is retransmitted, it might have a sequence
number after a message from epoch 1, even if the message from epoch 1
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RFC 6347 DTLS January 2012
was transmitted first. Note that some care needs to be taken during
the handshake to ensure that retransmitted messages use the right
epoch and keying material.
If several handshakes are performed in close succession, there might
be multiple records on the wire with the same sequence number but
from different cipher states. The epoch field allows recipients to
distinguish such packets. The epoch number is initially zero and is
incremented each time a ChangeCipherSpec message is sent. In order
to ensure that any given sequence/epoch pair is unique,
implementations MUST NOT allow the same epoch value to be reused
within two times the TCP maximum segment lifetime. In practice, TLS
implementations rarely rehandshake; therefore, we do not expect this
to be a problem.
Note that because DTLS records may be reordered, a record from epoch
1 may be received after epoch 2 has begun. In general,
implementations SHOULD discard packets from earlier epochs, but if
packet loss causes noticeable problems they MAY choose to retain
keying material from previous epochs for up to the default MSL
specified for TCP [TCP] to allow for packet reordering. (Note that
the intention here is that implementors use the current guidance from
the IETF for MSL, not that they attempt to interrogate the MSL that
the system TCP stack is using.) Until the handshake has completed,
implementations MUST accept packets from the old epoch.
Conversely, it is possible for records that are protected by the
newly negotiated context to be received prior to the completion of a
handshake. For instance, the server may send its Finished message
and then start transmitting data. Implementations MAY either buffer
or discard such packets, though when DTLS is used over reliable
transports (e.g., SCTP), they SHOULD be buffered and processed once
the handshake completes. Note that TLS's restrictions on when
packets may be sent still apply, and the receiver treats the packets
as if they were sent in the right order. In particular, it is still
impermissible to send data prior to completion of the first
handshake.
Note that in the special case of a rehandshake on an existing
association, it is safe to process a data packet immediately, even if
the ChangeCipherSpec or Finished messages have not yet been received
provided that either the rehandshake resumes the existing session or
that it uses exactly the same security parameters as the existing
association. In any other case, the implementation MUST wait for the
receipt of the Finished message to prevent downgrade attack.
As in TLS, implementations MUST either abandon an association or
rehandshake prior to allowing the sequence number to wrap.
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RFC 6347 DTLS January 2012
Similarly, implementations MUST NOT allow the epoch to wrap, but
instead MUST establish a new association, terminating the old
association as described in Section 4.2.8. In practice,
implementations rarely rehandshake repeatedly on the same channel, so
this is not likely to be an issue.
4.1.1. Transport Layer Mapping
Each DTLS record MUST fit within a single datagram. In order to
avoid IP fragmentation, clients of the DTLS record layer SHOULD
attempt to size records so that they fit within any PMTU estimates
obtained from the record layer.
Note that unlike IPsec, DTLS records do not contain any association
identifiers. Applications must arrange to multiplex between
associations. With UDP, this is presumably done with the host/port
number.
Multiple DTLS records may be placed in a single datagram. They are
simply encoded consecutively. The DTLS record framing is sufficient
to determine the boundaries. Note, however, that the first byte of
the datagram payload must be the beginning of a record. Records may
not span datagrams.
Some transports, such as DCCP [DCCP] provide their own sequence
numbers. When carried over those transports, both the DTLS and the
transport sequence numbers will be present. Although this introduces
a small amount of inefficiency, the transport layer and DTLS sequence
numbers serve different purposes; therefore, for conceptual
simplicity, it is superior to use both sequence numbers. In the
future, extensions to DTLS may be specified that allow the use of
only one set of sequence numbers for deployment in constrained
environments.
Some transports, such as DCCP, provide congestion control for traffic
carried over them. If the congestion window is sufficiently narrow,
DTLS handshake retransmissions may be held rather than transmitted
immediately, potentially leading to timeouts and spurious
retransmission. When DTLS is used over such transports, care should
be taken not to overrun the likely congestion window. [DCCPDTLS]
defines a mapping of DTLS to DCCP that takes these issues into
account.
4.1.1.1. PMTU Issues
In general, DTLS's philosophy is to leave PMTU discovery to the
application. However, DTLS cannot completely ignore PMTU for three
reasons:
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RFC 6347 DTLS January 2012
- The DTLS record framing expands the datagram size, thus lowering
the effective PMTU from the application's perspective.
- In some implementations, the application may not directly talk to
the network, in which case the DTLS stack may absorb ICMP
[RFC1191] "Datagram Too Big" indications or ICMPv6 [RFC4443]
"Packet Too Big" indications.
- The DTLS handshake messages can exceed the PMTU.
In order to deal with the first two issues, the DTLS record layer
SHOULD behave as described below.
If PMTU estimates are available from the underlying transport
protocol, they should be made available to upper layer protocols. In
particular:
- For DTLS over UDP, the upper layer protocol SHOULD be allowed to
obtain the PMTU estimate maintained in the IP layer.
- For DTLS over DCCP, the upper layer protocol SHOULD be allowed to
obtain the current estimate of the PMTU.
- For DTLS over TCP or SCTP, which automatically fragment and
reassemble datagrams, there is no PMTU limitation. However, the
upper layer protocol MUST NOT write any record that exceeds the
maximum record size of 2^14 bytes.
The DTLS record layer SHOULD allow the upper layer protocol to
discover the amount of record expansion expected by the DTLS
processing. Note that this number is only an estimate because of
block padding and the potential use of DTLS compression.
If there is a transport protocol indication (either via ICMP or via a
refusal to send the datagram as in Section 14 of [DCCP]), then the
DTLS record layer MUST inform the upper layer protocol of the error.
The DTLS record layer SHOULD NOT interfere with upper layer protocols
performing PMTU discovery, whether via [RFC1191] or [RFC4821]
mechanisms. In particular:
- Where allowed by the underlying transport protocol, the upper
layer protocol SHOULD be allowed to set the state of the DF bit
(in IPv4) or prohibit local fragmentation (in IPv6).
- If the underlying transport protocol allows the application to
request PMTU probing (e.g., DCCP), the DTLS record layer should
honor this request.
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RFC 6347 DTLS January 2012
The final issue is the DTLS handshake protocol. From the perspective
of the DTLS record layer, this is merely another upper layer
protocol. However, DTLS handshakes occur infrequently and involve
only a few round trips; therefore, the handshake protocol PMTU
handling places a premium on rapid completion over accurate PMTU
discovery. In order to allow connections under these circumstances,
DTLS implementations SHOULD follow the following rules:
- If the DTLS record layer informs the DTLS handshake layer that a
message is too big, it SHOULD immediately attempt to fragment it,
using any existing information about the PMTU.
- If repeated retransmissions do not result in a response, and the
PMTU is unknown, subsequent retransmissions SHOULD back off to a
smaller record size, fragmenting the handshake message as
appropriate. This standard does not specify an exact number of
retransmits to attempt before backing off, but 2-3 seems
appropriate.
4.1.2. Record Payload Protection
Like TLS, DTLS transmits data as a series of protected records. The
rest of this section describes the details of that format.
4.1.2.1. MAC
The DTLS MAC is the same as that of TLS 1.2. However, rather than
using TLS's implicit sequence number, the sequence number used to
compute the MAC is the 64-bit value formed by concatenating the epoch
and the sequence number in the order they appear on the wire. Note
that the DTLS epoch + sequence number is the same length as the TLS
sequence number.
TLS MAC calculation is parameterized on the protocol version number,
which, in the case of DTLS, is the on-the-wire version, i.e., {254,
253} for DTLS 1.2.
Note that one important difference between DTLS and TLS MAC handling
is that in TLS, MAC errors must result in connection termination. In
DTLS, the receiving implementation MAY simply discard the offending
record and continue with the connection. This change is possible
because DTLS records are not dependent on each other in the way that
TLS records are.
In general, DTLS implementations SHOULD silently discard records with
bad MACs or that are otherwise invalid. They MAY log an error. If a
DTLS implementation chooses to generate an alert when it receives a
message with an invalid MAC, it MUST generate a bad_record_mac alert
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RFC 6347 DTLS January 2012
with level fatal and terminate its connection state. Note that
because errors do not cause connection termination, DTLS stacks are
more efficient error type oracles than TLS stacks. Thus, it is
especially important that the advice in Section 6.2.3.2 of [TLS12] be
followed.
4.1.2.2. Null or Standard Stream Cipher
The DTLS NULL cipher is performed exactly as the TLS 1.2 NULL cipher.
The only stream cipher described in TLS 1.2 is RC4, which cannot be
randomly accessed. RC4 MUST NOT be used with DTLS.
4.1.2.3. Block Cipher
DTLS block cipher encryption and decryption are performed exactly as
with TLS 1.2.
4.1.2.4. AEAD Ciphers
TLS 1.2 introduced authenticated encryption with additional data
(AEAD) cipher suites. The existing AEAD cipher suites, defined in
[ECCGCM] and [RSAGCM], can be used with DTLS exactly as with TLS 1.2.
4.1.2.5. New Cipher Suites
Upon registration, new TLS cipher suites MUST indicate whether they
are suitable for DTLS usage and what, if any, adaptations must be
made (see Section 7 for IANA considerations).
4.1.2.6. Anti-Replay
DTLS records contain a sequence number to provide replay protection.
Sequence number verification SHOULD be performed using the following
sliding window procedure, borrowed from Section 3.4.3 of [ESP].
The receiver packet counter for this session MUST be initialized to
zero when the session is established. For each received record, the
receiver MUST verify that the record contains a sequence number that
does not duplicate the sequence number of any other record received
during the life of this session. This SHOULD be the first check
applied to a packet after it has been matched to a session, to speed
rejection of duplicate records.
Duplicates are rejected through the use of a sliding receive window.
(How the window is implemented is a local matter, but the following
text describes the functionality that the implementation must
exhibit.) A minimum window size of 32 MUST be supported, but a
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RFC 6347 DTLS January 2012
window size of 64 is preferred and SHOULD be employed as the default.
Another window size (larger than the minimum) MAY be chosen by the
receiver. (The receiver does not notify the sender of the window
size.)
The "right" edge of the window represents the highest validated
sequence number value received on this session. Records that contain
sequence numbers lower than the "left" edge of the window are
rejected. Packets falling within the window are checked against a
list of received packets within the window. An efficient means for
performing this check, based on the use of a bit mask, is described
in Section 3.4.3 of [ESP].
If the received record falls within the window and is new, or if the
packet is to the right of the window, then the receiver proceeds to
MAC verification. If the MAC validation fails, the receiver MUST
discard the received record as invalid. The receive window is
updated only if the MAC verification succeeds.
4.1.2.7. Handling Invalid Records
Unlike TLS, DTLS is resilient in the face of invalid records (e.g.,
invalid formatting, length, MAC, etc.). In general, invalid records
SHOULD be silently discarded, thus preserving the association;
however, an error MAY be logged for diagnostic purposes.
Implementations which choose to generate an alert instead, MUST
generate fatal level alerts to avoid attacks where the attacker
repeatedly probes the implementation to see how it responds to
various types of error. Note that if DTLS is run over UDP, then any
implementation which does this will be extremely susceptible to
denial-of-service (DoS) attacks because UDP forgery is so easy.
Thus, this practice is NOT RECOMMENDED for such transports.
If DTLS is being carried over a transport that is resistant to
forgery (e.g., SCTP with SCTP-AUTH), then it is safer to send alerts
because an attacker will have difficulty forging a datagram that will
not be rejected by the transport layer.
4.2. The DTLS Handshake Protocol
DTLS uses all of the same handshake messages and flows as TLS, with
three principal changes:
1. A stateless cookie exchange has been added to prevent denial-
of-service attacks.
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RFC 6347 DTLS January 2012
2. Modifications to the handshake header to handle message loss,
reordering, and DTLS message fragmentation (in order to avoid
IP fragmentation).
3. Retransmission timers to handle message loss.
With these exceptions, the DTLS message formats, flows, and logic are
the same as those of TLS 1.2.
4.2.1. Denial-of-Service Countermeasures
Datagram security protocols are extremely susceptible to a variety of
DoS attacks. Two attacks are of particular concern:
1. An attacker can consume excessive resources on the server by
transmitting a series of handshake initiation requests, causing
the server to allocate state and potentially to perform
expensive cryptographic operations.
2. An attacker can use the server as an amplifier by sending
connection initiation messages with a forged source of the
victim. The server then sends its next message (in DTLS, a
Certificate message, which can be quite large) to the victim
machine, thus flooding it.
In order to counter both of these attacks, DTLS borrows the stateless
cookie technique used by Photuris [PHOTURIS] and IKE [IKEv2]. When
the client sends its ClientHello message to the server, the server
MAY respond with a HelloVerifyRequest message. This message contains
a stateless cookie generated using the technique of [PHOTURIS]. The
client MUST retransmit the ClientHello with the cookie added. The
server then verifies the cookie and proceeds with the handshake only
if it is valid. This mechanism forces the attacker/client to be able
to receive the cookie, which makes DoS attacks with spoofed IP
addresses difficult. This mechanism does not provide any defense
against DoS attacks mounted from valid IP addresses.
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RFC 6347 DTLS January 2012
The exchange is shown below:
Client Server
------ ------
ClientHello ------>
<----- HelloVerifyRequest
(contains cookie)
ClientHello ------>
(with cookie)
[Rest of handshake]
DTLS therefore modifies the ClientHello message to add the cookie
value.
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
opaque cookie<0..2^8-1>; // New field
CipherSuite cipher_suites<2..2^16-1>;
CompressionMethod compression_methods<1..2^8-1>;
} ClientHello;
When sending the first ClientHello, the client does not have a cookie
yet; in this case, the Cookie field is left empty (zero length).
The definition of HelloVerifyRequest is as follows:
struct {
ProtocolVersion server_version;
opaque cookie<0..2^8-1>;
} HelloVerifyRequest;
The HelloVerifyRequest message type is hello_verify_request(3).
The server_version field has the same syntax as in TLS. However, in
order to avoid the requirement to do version negotiation in the
initial handshake, DTLS 1.2 server implementations SHOULD use DTLS
version 1.0 regardless of the version of TLS that is expected to be
negotiated. DTLS 1.2 and 1.0 clients MUST use the version solely to
indicate packet formatting (which is the same in both DTLS 1.2 and
1.0) and not as part of version negotiation. In particular, DTLS 1.2
clients MUST NOT assume that because the server uses version 1.0 in
the HelloVerifyRequest that the server is not DTLS 1.2 or that it
will eventually negotiate DTLS 1.0 rather than DTLS 1.2.
Rescorla & Modadugu Standards Track [Page 16]

RFC 6347 DTLS January 2012
When responding to a HelloVerifyRequest, the client MUST use the same
parameter values (version, random, session_id, cipher_suites,
compression_method) as it did in the original ClientHello. The
server SHOULD use those values to generate its cookie and verify that
they are correct upon cookie receipt. The server MUST use the same
version number in the HelloVerifyRequest that it would use when
sending a ServerHello. Upon receipt of the ServerHello, the client
MUST verify that the server version values match. In order to avoid
sequence number duplication in case of multiple HelloVerifyRequests,
the server MUST use the record sequence number in the ClientHello as
the record sequence number in the HelloVerifyRequest.
Note: This specification increases the cookie size limit to 255 bytes
for greater future flexibility. The limit remains 32 for previous
versions of DTLS.
The DTLS server SHOULD generate cookies in such a way that they can
be verified without retaining any per-client state on the server.
One technique is to have a randomly generated secret and generate
cookies as:
Cookie = HMAC(Secret, Client-IP, Client-Parameters)
When the second ClientHello is received, the server can verify that
the Cookie is valid and that the client can receive packets at the
given IP address. In order to avoid sequence number duplication in
case of multiple cookie exchanges, the server MUST use the record
sequence number in the ClientHello as the record sequence number in
its initial ServerHello. Subsequent ServerHellos will only be sent
after the server has created state and MUST increment normally.
One potential attack on this scheme is for the attacker to collect a
number of cookies from different addresses and then reuse them to
attack the server. The server can defend against this attack by
changing the Secret value frequently, thus invalidating those
cookies. If the server wishes that legitimate clients be able to
handshake through the transition (e.g., they received a cookie with
Secret 1 and then sent the second ClientHello after the server has
changed to Secret 2), the server can have a limited window during
which it accepts both secrets. [IKEv2] suggests adding a version
number to cookies to detect this case. An alternative approach is
simply to try verifying with both secrets.
DTLS servers SHOULD perform a cookie exchange whenever a new
handshake is being performed. If the server is being operated in an
environment where amplification is not a problem, the server MAY be
configured not to perform a cookie exchange. The default SHOULD be
that the exchange is performed, however. In addition, the server MAY
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choose not to do a cookie exchange when a session is resumed.
Clients MUST be prepared to do a cookie exchange with every
handshake.
If HelloVerifyRequest is used, the initial ClientHello and
HelloVerifyRequest are not included in the calculation of the
handshake_messages (for the CertificateVerify message) and
verify_data (for the Finished message).
If a server receives a ClientHello with an invalid cookie, it SHOULD
treat it the same as a ClientHello with no cookie. This avoids
race/deadlock conditions if the client somehow gets a bad cookie
(e.g., because the server changes its cookie signing key).
Note to implementors: This may result in clients receiving multiple
HelloVerifyRequest messages with different cookies. Clients SHOULD
handle this by sending a new ClientHello with a cookie in response to
the new HelloVerifyRequest.
4.2.2. Handshake Message Format
In order to support message loss, reordering, and message
fragmentation, DTLS modifies the TLS 1.2 handshake header:
struct {
HandshakeType msg_type;
uint24 length;
uint16 message_seq; // New field
uint24 fragment_offset; // New field
uint24 fragment_length; // New field
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case hello_verify_request: HelloVerifyRequest; // New type
case server_hello: ServerHello;
case certificate:Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done:ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
} body;
} Handshake;
The first message each side transmits in each handshake always has
message_seq = 0. Whenever each new message is generated, the
message_seq value is incremented by one. Note that in the case of a
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rehandshake, this implies that the HelloRequest will have message_seq
= 0 and the ServerHello will have message_seq = 1. When a message is
retransmitted, the same message_seq value is used. For example:
Client Server
------ ------
ClientHello (seq=0) ------>
X<-- HelloVerifyRequest (seq=0)
(lost)
[Timer Expires]
ClientHello (seq=0) ------>
(retransmit)
<------ HelloVerifyRequest (seq=0)
ClientHello (seq=1) ------>
(with cookie)
<------ ServerHello (seq=1)
<------ Certificate (seq=2)
<------ ServerHelloDone (seq=3)
[Rest of handshake]
Note, however, that from the perspective of the DTLS record layer,
the retransmission is a new record. This record will have a new
DTLSPlaintext.sequence_number value.
DTLS implementations maintain (at least notionally) a
next_receive_seq counter. This counter is initially set to zero.
When a message is received, if its sequence number matches
next_receive_seq, next_receive_seq is incremented and the message is
processed. If the sequence number is less than next_receive_seq, the
message MUST be discarded. If the sequence number is greater than
next_receive_seq, the implementation SHOULD queue the message but MAY
discard it. (This is a simple space/bandwidth tradeoff).
4.2.3. Handshake Message Fragmentation and Reassembly
As noted in Section 4.1.1, each DTLS message MUST fit within a single
transport layer datagram. However, handshake messages are
potentially bigger than the maximum record size. Therefore, DTLS
provides a mechanism for fragmenting a handshake message over a
number of records, each of which can be transmitted separately, thus
avoiding IP fragmentation.
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When transmitting the handshake message, the sender divides the
message into a series of N contiguous data ranges. These ranges MUST
NOT be larger than the maximum handshake fragment size and MUST
jointly contain the entire handshake message. The ranges SHOULD NOT
overlap. The sender then creates N handshake messages, all with the
same message_seq value as the original handshake message. Each new
message is labeled with the fragment_offset (the number of bytes
contained in previous fragments) and the fragment_length (the length
of this fragment). The length field in all messages is the same as
the length field of the original message. An unfragmented message is
a degenerate case with fragment_offset=0 and fragment_length=length.
When a DTLS implementation receives a handshake message fragment, it
MUST buffer it until it has the entire handshake message. DTLS
implementations MUST be able to handle overlapping fragment ranges.
This allows senders to retransmit handshake messages with smaller
fragment sizes if the PMTU estimate changes.
Note that as with TLS, multiple handshake messages may be placed in
the same DTLS record, provided that there is room and that they are
part of the same flight. Thus, there are two acceptable ways to pack
two DTLS messages into the same datagram: in the same record or in
separate records.
4.2.4. Timeout and Retransmission
DTLS messages are grouped into a series of message flights, according
to the diagrams below. Although each flight of messages may consist
of a number of messages, they should be viewed as monolithic for the
purpose of timeout and retransmission.
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The state machine has three basic states.
In the PREPARING state, the implementation does whatever computations
are necessary to prepare the next flight of messages. It then
buffers them up for transmission (emptying the buffer first) and
enters the SENDING state.
In the SENDING state, the implementation transmits the buffered
flight of messages. Once the messages have been sent, the
implementation then enters the FINISHED state if this is the last
flight in the handshake. Or, if the implementation expects to
receive more messages, it sets a retransmit timer and then enters the
WAITING state.
There are three ways to exit the WAITING state:
1. The retransmit timer expires: the implementation transitions to
the SENDING state, where it retransmits the flight, resets the
retransmit timer, and returns to the WAITING state.
2. The implementation reads a retransmitted flight from the peer: the
implementation transitions to the SENDING state, where it
retransmits the flight, resets the retransmit timer, and returns
to the WAITING state. The rationale here is that the receipt of a
duplicate message is the likely result of timer expiry on the peer
and therefore suggests that part of one's previous flight was
lost.
3. The implementation receives the next flight of messages: if this
is the final flight of messages, the implementation transitions to
FINISHED. If the implementation needs to send a new flight, it
transitions to the PREPARING state. Partial reads (whether
partial messages or only some of the messages in the flight) do
not cause state transitions or timer resets.
Because DTLS clients send the first message (ClientHello), they start
in the PREPARING state. DTLS servers start in the WAITING state, but
with empty buffers and no retransmit timer.
When the server desires a rehandshake, it transitions from the
FINISHED state to the PREPARING state to transmit the HelloRequest.
When the client receives a HelloRequest, it transitions from FINISHED
to PREPARING to transmit the ClientHello.
In addition, for at least twice the default MSL defined for [TCP],
when in the FINISHED state, the node that transmits the last flight
(the server in an ordinary handshake or the client in a resumed
handshake) MUST respond to a retransmit of the peer's last flight
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with a retransmit of the last flight. This avoids deadlock
conditions if the last flight gets lost. This requirement applies to
DTLS 1.0 as well, and though not explicit in [DTLS1], it was always
required for the state machine to function correctly. To see why
this is necessary, consider what happens in an ordinary handshake if
the server's Finished message is lost: the server believes the
handshake is complete but it actually is not. As the client is
waiting for the Finished message, the client's retransmit timer will
fire and it will retransmit the client's Finished message. This will
cause the server to respond with its own Finished message, completing
the handshake. The same logic applies on the server side for the
resumed handshake.
Note that because of packet loss, it is possible for one side to be
sending application data even though the other side has not received
the first side's Finished message. Implementations MUST either
discard or buffer all application data packets for the new epoch
until they have received the Finished message for that epoch.
Implementations MAY treat receipt of application data with a new
epoch prior to receipt of the corresponding Finished message as
evidence of reordering or packet loss and retransmit their final
flight immediately, shortcutting the retransmission timer.
4.2.4.1. Timer Values
Though timer values are the choice of the implementation, mishandling
of the timer can lead to serious congestion problems; for example, if
many instances of a DTLS time out early and retransmit too quickly on
a congested link. Implementations SHOULD use an initial timer value
of 1 second (the minimum defined in RFC 6298 [RFC6298]) and double
the value at each retransmission, up to no less than the RFC 6298
maximum of 60 seconds. Note that we recommend a 1-second timer
rather than the 3-second RFC 6298 default in order to improve latency
for time-sensitive applications. Because DTLS only uses
retransmission for handshake and not dataflow, the effect on
congestion should be minimal.
Implementations SHOULD retain the current timer value until a
transmission without loss occurs, at which time the value may be
reset to the initial value. After a long period of idleness, no less
than 10 times the current timer value, implementations may reset the
timer to the initial value. One situation where this might occur is
when a rehandshake is used after substantial data transfer.
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RFC 6347 DTLS January 20124.2.5. ChangeCipherSpec
As with TLS, the ChangeCipherSpec message is not technically a
handshake message but MUST be treated as part of the same flight as
the associated Finished message for the purposes of timeout and
retransmission. This creates a potential ambiguity because the order
of the ChangeCipherSpec cannot be established unambiguously with
respect to the handshake messages in case of message loss.
This is not a problem with any current TLS mode because the expected
set of handshake messages logically preceeding the ChangeCipherSpec
is predictable from the rest of the handshake state. However, future
modes MUST take care to avoid creating ambiguity.
4.2.6. CertificateVerify and Finished Messages
CertificateVerify and Finished messages have the same format as in
TLS. Hash calculations include entire handshake messages, including
DTLS-specific fields: message_seq, fragment_offset, and
fragment_length. However, in order to remove sensitivity to
handshake message fragmentation, the Finished MAC MUST be computed as
if each handshake message had been sent as a single fragment. Note
that in cases where the cookie exchange is used, the initial
ClientHello and HelloVerifyRequest MUST NOT be included in the
CertificateVerify or Finished MAC computations.
4.2.7. Alert Messages
Note that Alert messages are not retransmitted at all, even when they
occur in the context of a handshake. However, a DTLS implementation
which would ordinarily issue an alert SHOULD generate a new alert
message if the offending record is received again (e.g., as a
retransmitted handshake message). Implementations SHOULD detect when
a peer is persistently sending bad messages and terminate the local
connection state after such misbehavior is detected.
4.2.8. Establishing New Associations with Existing Parameters
If a DTLS client-server pair is configured in such a way that
repeated connections happen on the same host/port quartet, then it is
possible that a client will silently abandon one connection and then
initiate another with the same parameters (e.g., after a reboot).
This will appear to the server as a new handshake with epoch=0. In
cases where a server believes it has an existing association on a
given host/port quartet and it receives an epoch=0 ClientHello, it
SHOULD proceed with a new handshake but MUST NOT destroy the existing
association until the client has demonstrated reachability either by
completing a cookie exchange or by completing a complete handshake
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The primary additional security consideration raised by DTLS is that
of denial of service. DTLS includes a cookie exchange designed to
protect against denial of service. However, implementations that do
not use this cookie exchange are still vulnerable to DoS. In
particular, DTLS servers that do not use the cookie exchange may be
used as attack amplifiers even if they themselves are not
experiencing DoS. Therefore, DTLS servers SHOULD use the cookie
exchange unless there is good reason to believe that amplification is
not a threat in their environment. Clients MUST be prepared to do a
cookie exchange with every handshake.
Unlike TLS implementations, DTLS implementations SHOULD NOT respond
to invalid records by terminating the connection. See Section4.1.2.7 for details on this.
6. Acknowledgments
The authors would like to thank Dan Boneh, Eu-Jin Goh, Russ Housley,
Constantine Sapuntzakis, and Hovav Shacham for discussions and
comments on the design of DTLS. Thanks to the anonymous NDSS
reviewers of our original NDSS paper on DTLS [DTLS] for their
comments. Also, thanks to Steve Kent for feedback that helped
clarify many points. The section on PMTU was cribbed from the DCCP
specification [DCCP]. Pasi Eronen provided a detailed review of this
specification. Peter Saint-Andre provided the list of changes in
Section 8. Helpful comments on the document were also received from
Mark Allman, Jari Arkko, Mohamed Badra, Michael D'Errico, Adrian
Farrell, Joel Halpern, Ted Hardie, Charlia Kaufman, Pekka Savola,
Allison Mankin, Nikos Mavrogiannopoulos, Alexey Melnikov, Robin
Seggelmann, Michael Tuexen, Juho Vaha-Herttua, and Florian Weimer.
7. IANA Considerations
This document uses the same identifier space as TLS [TLS12], so no
new IANA registries are required. When new identifiers are assigned
for TLS, authors MUST specify whether they are suitable for DTLS.
IANA has modified all TLS parameter registries to add a DTLS-OK flag,
indicating whether the specification may be used with DTLS. At the
time of publication, all of the [TLS12] registrations except the
following are suitable for DTLS. The full table of registrations is
available at [IANA].
From the TLS Cipher Suite Registry:
0x00,0x03 TLS_RSA_EXPORT_WITH_RC4_40_MD5 [RFC4346]
0x00,0x04 TLS_RSA_WITH_RC4_128_MD5 [RFC5246]
0x00,0x05 TLS_RSA_WITH_RC4_128_SHA [RFC5246]
0x00,0x17 TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 [RFC4346]
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